If you were lucky enough to get your mits on one of the few Radeon 5800 cards that were available last week, then you will need this. You may have found yourself unable to install the latest edition of the catalyst drivers - 9.9, as it doesn't officially support the Cypress chip yet.
AMD have come out with this hotfix, which will tide users over until the release of Catalyst 9.10, which will bring full compatibility in a single package. The hotfix supports both the 5850 and 5870, and is intended for use with Windows Vista and 7, in 32bit and 64bit varieties. You can download it directly from AMD here